Axial flow turbofan gas turbine engines are well known and widely used in the air transport industry. Simply put, an axial turbofan engine differs from an axial turbojet engine in that a portion of the air compressed by the beginning compressor stages, often termed the "fan section", is bypassed coaxially around the inner gas generator or core. This bypass fan air flows for at least some distance in an annular passage created between the gas generator casing and an outer, coaxial fan duct.
In modern axial flow turbofan engines, it is commonly desirable to effect certain mechanical adjustments to various engine structures or components during operation. Such structures include the radially oriented compressor stator vanes, disposed in the compressor section of the gas generator in one or more axially spaced apart sets termed "stages". For reasons of engine operability, reliability, and power output, it is occasionally desirable to simultaneously alter the angle of attack of the generally axially flowing airstream encountering an individual set of stator vanes. Such adjustment is typically carried out by furnishing each individual vane with a mount rotatable about a radially oriented axis, linking each blade of an individual stage together by a plurality of corresponding vane arms extending perpendicular to each axis of rotation for each blade, each arm further being joined at the end thereof to a unison ring encircling the generally cylindrical compressor case and causing equal radial rotation in each linked stator vane in response to relative circumferential displacement between the unison ring and the compressor case.
Such unison ring-variable stator vane arrangements are well known in the gas turbine engine art, requiring only the addition of a selectively drivable actuator and connecting linkage to the above described system to result in an operable system. Certain of these systems known in the prior art use an actuator and driving linkage secured to the compressor casing and disposed wholly within the annular flow passage formed in conjunction with the fan duct. Certain other turbofan engine arrangements, either of small size or having a reduced percentage of the total incoming air bypassed into the passage, have insufficient volume to allow positioning of an actuator and a linkage between the compresor casing and the fan duct without undesirably disrupting airflow therein or hampering field maintenance personnel.
In such engines having insufficient clearance for installing an actuator between the compressor case and surrounding fan duct, it is commonplace to mount the actuator on the exterior of the fan duct and to connect the actuator to the unison ring or other internal driven structure by a mechanical linkage. In addition to manipulating the internal engine structure in response to the externally mounted actuator, such linkages must also accommodate differential or relative movement which frequently occurs between the fan duct and compressor case. Such relative movement, caused by thermal transients, engine loading, externally induced forces, etc. may occur in the axial, circumferential, or radial direction either individually or in combination.
Prior art methods of accommodating this relative movement while still linking the external actuator and internal structure have included the use of corresponding, slidably engaged, splined shaft members disposed between the internal structure and the external actuator. Such slidably engaged structures with their several components have proved less satisfactory in those applications wherein it is desirable to manipulate the internal engine structure to a high degree of accuracy. Such multi-component structures have tended to wear at the sliding interfaces, resulting in a linkage backlash or hysteresis which can degrade the positional accuracy of the internal structure. Such accuracy is particularly important when positioning stator vanes in order to optimize compressor performance and operational reliability.